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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Plumbing Questions >> Who's right ? https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1515067931 Message started by londonman on Jan 4^th, 2018, 12:12pm

Title: Who's right ?
Post by londonman on Jan 4^th, 2018, 12:12pm

Can someone please confirm that I am correct in my thinking ?

The first photo is a rough drawing of how the CH layout used to work. The pump was located in the garage by the heat store. The pump fed a metre into the house where the feed split to two motorised valves. One valve sent the hot water down a dedicated pipe to the hot water cylinder. The second valve fed the CH feed and about another metre after this valve it split to feed the upstairs rads and downstairs rads. Within feet of where the split occurred, this feed pipe was hitting the radiators that it was designed to feed. Assuming that the system was reasonably balanced then my argument is that the temperature in this feed pipe is going to be roughly that when it left the heat store...give or take the odd degree.

NB the diagram does not show the CH split to the downstairs rads.

Now when my plumber put in the new boiler he was very fixated on neutral points and insisted that the pump be relocated up at the hot water tank and the feed and vent pipes connected there to create the neutral point. However this meant that the motorised valves needed to be located here as well which in turn necessitated modifications to the primary hot water feed.

The two original valves were removed and the T between HW and rads removed. The feed to the rads was cut off and blocked. The original and remaining feed to the HW tank became the main feed to the new pumps' location. Two new valves after the pump...the one we're interested in...the CH rad one then feeding a new pipe that T'd in part way down the upstairs original CH feed run.

With this new arrangement, I am 11 degrees of heat lower at the pump then that leaving the heat store because of the long run now built in to the system due to the new layout. 66 degrees leaving the heat store (don't worry about whether this is high enough etc for the purposes of this discussion) and 55 degrees when it hits the actual CH upstairs rad circuit. My plumber does not accept this. He says that I'd see the same thing on the original system which I think is bonkers. Original system...distance between heat store and first rad ...4m. New system...distance about 30m.

A pressurised system was and is out of the question as there was existing inaccessible pipework underneath the floorboards with joints of unknown provenance and as I'd already discovered one joint that wasn't properly soldered, I think this decision is sound.

Supplementary question. The furthest radiators from the CH feed point are in the kitchen. With every other radiator and HW tank off, the hottest temperature of the incoming CH water to these rads is a meagre 36 degrees. Is this an indication of an under-powered pump or a blockage in the old pipework ?

Second supplementary question. My plumber says that you get different temperature readings if the pipe is vertical or horizontal. EG horizontal copper pipe coming to a 90 degree feeding the vertical pipe into the radiator tail. Doesn't make sense to me.

Title: Re: Who's right ?
Post by thescruff on Jan 5^th, 2018, 1:39am

Crikey my head hurts reading that :-?

Can you do a better line drawing showing the connections from the cylinder to the pump and then the rads.

You mention heat store a few times, is it a cylinder or thermal store (make and model) please.

Title: Re: Who's right ?
Post by londonman on Jan 5^th, 2018, 4:43pm

My drawings are lousy! Just think of the flow as coming from the heat store (Akvaterm) then up that vertical red arrow, down the pink arrow to the hw cylinder area. There it goes to the pump and after the pump the usual split CH/HW with two motorised Honeywell valves. The fuzzy grey line is the CH feed after the Honeywell valve and it connects into the original CH system flow halfway down.

It's a tad academic now as he came round yesterday and he and his son spent a good 3-4 hours getting rid of the air in the system. Involving hosepipes and suction pumps. This air was partially blocking the return...once it was out of the way the whole system sprung to life with good heat in all radiators.

We had the heat loss discussion. I came at it by describing a system where there was 500 yards of copper pipe between heat store and the first rad. Eventually he agreed with me. But we then had the further discussion about the discussions we'd had at the outset. There were the three options - as he reminded me.

Option A - keep the pump at the heat store and run two pipes down to it to create the neutral point. Trouble was that given all the steel etc in the building getting a clean run back - especially for the vent pipe that needed a gradual upward slope - was nigh on impossible.

Option B - the one we went for. Move the pump up to the area of the HW tank (where the feed and vent pipes were), make the tweaks to the circuit as outlined above. The option we went for. What I am a bit annoyed about is that he didn't point out to me the implications re heat loss in the changed circuit - especially as it was he who suggested the use of the heat store to run the boiler fully in condensing mode etc. This heat loss may mean that I can't run the boiler as efficiently as I wanted to and I've wasted all that money. Time will tell. He suggested that with the improved flow rate that it might not be as bad as it was. Fingers crossed.

Had I known about the heat loss then I guess I could have tried a bit harder to find a way to go the Option A route. Hard call. I do know that I tried damn hard the first time round (it was possible pump noise that was the driver as LOML has the hearing of a barn owl).

Option C - a pressurised system. Which, as previously mentioned, was ruled out due to unknown existing pipework provenance. We shall see...may have to bite the bullet and go for it downline.

That was yesterday and all tickety boo. Then today I needed to turn the pump off. When I turned it back on, the old symptoms came back so there is clearly still air in the return. Bit bloody annoying as we had good heat from upstairs and downstairs rads but now downstairs are cold and upstairs tepid at best.

Anyway, if you're still with us (!) back to that heat loss. If I got a pump with a faster flow rate then the water wouldn't lose as much heat? Yes? No ?

Title: Re: Who's right ?
Post by thescruff on Jan 5^th, 2018, 9:03pm

Is there a lockshield valve on the store return because it looks as though that circuit would pinch the lions share of the flow.

Have/can you try the heating only without the store being on.

Trying to understand it is difficult and certainly, I would say a likely problem as he's restricted the heating flow ( I think)

Title: Re: Who's right ?
Post by thescruff on Jan 5^th, 2018, 9:10pm

One other thing you should try is holding a bottle of water under the vent pipe and get someone to turn the pumps on and off.

I only wish you had asked an opinion about sludge buckets, (thermal stores) I don't think you would have gone for it.

Insulation in unheated spaces is critical to any system.

If you getting air that can't be removed it hasn't been installed correctly.

Title: Re: Who's right ?
Post by londonman on Jan 6^th, 2018, 8:06am

The heat store provides the heat so can't be taken out of the equation! Not a new system but an existing one. Just that he's changed things a bit!

Title: Re: Who's right ?
Post by thescruff on Jan 6^th, 2018, 10:15am

Yes, I realise that, but it looks like the store circuit is the easy way back.

It had an MV before so there was some control on it and assume that is still the case, but is there a valve on the return you can adjust, To balance the circuit a bit

Title: Re: Who's right ?
Post by londonman on Jan 6^th, 2018, 4:53pm

The heat store is the only way back. Just think of it as a central heating boiler.

Title: Re: Who's right ?
Post by thescruff on Jan 6^th, 2018, 8:59pm

Yes I know that but the store return and heating return have to be at least common somewhere in the circuit

Title: Re: Who's right ?
Post by thescruff on Jan 6^th, 2018, 9:03pm

Can you pencil a sketch and add flow arrows and id the pipes

Title: Re: Who's right ?
Post by londonman on Jan 7^th, 2018, 9:16am

thescruff wrote on Jan 6^th, 2018, 8:59pm:

Yes I know that but the store return and heating return have to be at least common somewhere in the circuit

Not sure what you mean by the 'store return'. I have two separate heat exchanger coils inside the heat store. One of them comes from the oil boiler which heats up the water. The other feeds the ch/hw circuit

Title: Re: Who's right ?
Post by thescruff on Jan 7^th, 2018, 9:18pm

In your 2nd drawing you show a red pipe heading back to the store and a pink pipe going I assume the heating.

Where it has been modified the pump looks to connect to the red pipe.

Is the red pipe now the flow and where does each end go.

What happened to the pink pipe and where did it go.

Title: Re: Who's right ?
Post by CWatters on Jan 8^th, 2018, 4:49pm

I'm late to the party but it seems odd to me to have a heat store heating a hot water cylinder. Normally you would just have a coil in or heat exchanger on the heat store to heat DHW.

Title: Re: Who's right ?
Post by londonman on Jan 8^th, 2018, 9:57pm

thescruff wrote on Jan 7^th, 2018, 9:18pm:

I see the confusion. My bad. Sorry.

Ok..in the original system the hot water from the store split into the two circuits - HW and CH. In drawing two, that red arrow going approx 10 o'clock is left over and should not be in the drawing. The plumber has used what was the original feed pipe to the HW cylinder as the pipe to get the hot water from the store and into the house where the new pump is located...just to get his bloody neutral point and to keep him happy.

So at the end of that pink pipe he's broken it away from where it was feeding the hot water cylinder and it now goes to the pump at its new location. After the pump the usual two motorised valves are there ....splitting the hot water flow to the HW cylinder and radiators. The new connection to the radiator circuit is that grey pipe. So the hot water now injects back into what was the CH main flow pipe.

I can't recall how the original two returns came back together prior to returning to the heat store. Now hidden behind plaster.
I'm now not bothered about the heat loss as I'd like to stop the bloody thing from stopping working.

Turned the pump off to do some work. After switching it back on...no heat downstairs and upstairs rads lukewarm. Fiddled about with a hose pipe in the bleed valve orifice and finally got the system up and running again.

Now today, I put on the TRVs which had the effect of turning the pump off ...as it were...as the sodding system has stopped heating again. Tried the hosepipe bleed this evening but failed miserably. There is an airlock keeps getting introduced into the return pipe(s) somehow.

Am I allowed to say I hate this f***ing system ?

Title: Re: Who's right ?
Post by londonman on Jan 8^th, 2018, 9:58pm

CWatters wrote on Jan 8^th, 2018, 4:49pm:

I'm late to the party but it seems odd to me to have a heat store heating a hot water cylinder. Normally you would just have a coil in or heat exchanger on the heat store to heat DHW.

Red herring. Don;t go there.

Title: Re: Who's right ?
Post by thescruff on Jan 9^th, 2018, 1:24am

Back too one of my earlier post.

You have a header tank for the heating, hold a jar of water under the vent, turn the pump on and off a few times, does it drink the water or blow bubbles.

Can you post a pic of the neutral point.

Title: Re: Who's right ?
Post by londonman on Jan 9^th, 2018, 10:33am

Many thanks for your continued support, Scruff.

Here is a photo of the neutral point - located upstairs landing

Be a while before I can get help to turn the pump on/off while I am up in the loft

Title: Re: Who's right ?
Post by thescruff on Jan 9^th, 2018, 12:41pm

That looks 1st class, one should assume he knows what he's doing.

Ask the guy where the air is coming from and what he can do. Most of the air should be out by now.

Check the header tank when the heating is on, is the ball valve dribbling or pumping over the vent.

Title: Re: Who's right ?
Post by londonman on Jan 9^th, 2018, 4:33pm

Yes, I thought so to.

On another forum I was asked some very cogent questions, and some that I couldn't answer. For example, why would air 'block' the flow ? So I did a bit of Googling to find out why and came across this...

In a well designed low pressure water pipe system, air should clear naturally even if you run out of water. As the pipe system fills again, the air should naturally rise to the top and be expelled. Air locks happen when the pipe system is not well designed.

Air bubbles rise to the top because air is much lighter than water. In a correctly installed low pressure pipe there is a natural "fall" towards the taps. Friction between the water and the pipe walls provides a resistance to the flow. The fall in the pipe is needed to overcome the resistance.

If the fall in the pipe is continuous, any trapped air going the other way in the pipe will be following a natural rise. Given a little time it should bubble back into the cold tank or, if it's a hot water pipe, bubble back towards the cylinder and up the open vent over the cold tank.

Resistance increases with the length of the pipe and long horizontal sections can be a problem because they have no fall. Short horizontal sections of pipe are not normally a problem.

And therein lies the problem, I think. There are two pairs of 22mm pipes that run 'horizontally' down the length of the house. One pair were the flow and return for the radiators and the other air the flow and return to the HW cylinder. I don't recall exactly but I have a vague recollection that they may well have risen slightly from the old pump end.

Now with the new modified layout we have a bit of a rollercoaster. I wonder if this is the cause ?

Having aid that the cure has always been to bleed the return side of things and we've not touched that but the fact remains that for part of the radiator circuit the flow is in the opposite direction to what it was and most likely downhill.

The article then goes on...although I admit I don't follow it 100%

Pipes which rise towards the taps are a big problem. However, we can ignore the last short section of pipe connecting to the tap which nearly always rises vertically. Any air going to the top of that section is simply passed out through the tap. If a low pressure pipe rises on its way towards the taps, any air going the other way would have to go downhill. Because air is lighter than water it gets trapped at the top of the rise in the pipe. This air pocket adds a lot of resistance to the flow of water. If there is not sufficient overall fall along the length of the pipe to overcome that extra resistance, the pipe becomes air locked.
When an air lock forms, the flow of water from the taps may reduce dramatically and can sometimes stop completely. If the air lock is in a cold supply pipe it may also stop the loo filling.

Title: Re: Who's right ?
Post by thescruff on Jan 9^th, 2018, 6:47pm

where the pipes rise there should be air vents to take the air out at high levels.

You'd need to see it to decide where is best.

Title: Re: Who's right ?
Post by londonman on Jan 9^th, 2018, 9:08pm

Mmm...intriguing. Thing is the rad that I bleed to fix the problem is the one furthest away from the pump ie 'downhill'.

Title: Re: Who's right ?
Post by thescruff on Jan 10^th, 2018, 8:46pm

If the air gets to the rads then it is likely to collect the air and you may find one rad collects more than the others. that is normal although once the air is out it should stay out.

Have you got an inhibitor in the system.

Title: Re: Who's right ?
Post by londonman on Jan 11^th, 2018, 1:00pm

thescruff wrote on Jan 10^th, 2018, 8:46pm:

Yes I do but given the amount of bleeding I will be topping it up.

Title: Re: Who's right ?
Post by thescruff on Jan 12^th, 2018, 12:29pm

give or take 2-3 weeks all the air should be out unless you have a system fault, which is why I'd like you to check the header tank as per my earlier posts.

Title: Re: Who's right ?
Post by londonman on Jan 13^th, 2018, 11:46am

thescruff wrote on Jan 9^th, 2018, 1:24am:

OK.

Switch the pump off and air bubbles come out.

Pump back on and the water gets sucked into the vent pipe.

I'm not sure what I was expecting....part of me was thinking nothing hence the 'neutral point'.

EDIT: OK, think I understand what's happening and it is as expected. We've got a body of water circulating by virtue of the pump running. When the pump stops, the momentum of that water pushed up the vent pipe a bit ...pushing out air in the downspout of the vent pipe (the bit hanging over the header tank).

When the pump starts up again there is a momentary low pressure at the neutral point as the pump starts sucking and the inertia of the water in the CH circuit takes a tad to get moving again. So that sucks water into the vent pipe.

Ergo the pump is running the right way round.

Do I get a gold star ? ;D

Title: Re: Who's right ?
Post by thescruff on Jan 13^th, 2018, 10:06pm

No gold star only for effort and finding a problem.

The neutral point isn't neutral as nothing should happen.

The Plumber needs to come back and sort it.

If the engineer wants help get him to improve on the sketch.

Title: Re: Who's right ?
Post by thescruff on Jan 13^th, 2018, 10:16pm

Separate post to avoid confusion.

A heating pump doesn't pump, it circulates at a high velocity, You would find near as a 1bar pressure differential across the inlet and outlet of the pump.

The bubbles when the pump stops is because the pump is sucking air in when it's running

Title: Re: Who's right ?
Post by londonman on Jan 13^th, 2018, 10:22pm

thescruff wrote on Jan 13^th, 2018, 10:16pm:

Separate post to avoid confusion.

......
The bubbles when the pump stops is because the pump is sucking air in when it's running

Can you please clarify this one for me as I'm struggling to understand this. Are you saying that there is constant air being drawn into the system and escaping via the vent pipe while the pump is running ?

Title: Re: Who's right ?
Post by thescruff on Jan 13^th, 2018, 10:30pm

Sort of, you have a problem with air in the system, now you know where it's coming from.

Maybe the pump is at the wrong speed, the connections are wrong, the tank and or vent is too low, but that should make any difference if the neutral point is correct.

To comment further one needs information, and preferably a good as fitted drawing, or seeing the job which is unlikely.

If it wasn't new one would say the cold feed is blocked.

Title: Re: Who's right ?
Post by thescruff on Jan 13^th, 2018, 10:58pm

[gallery fullsize]thescruff/1515884071.jpg[/gallery]

A little fag package sketch I knocked out for you.

Cold feed and vent should be as close as possible. When the pump starts and shortfall should take water from the cold feed and NOT the vent, doing the later highlights a problem, normally a blocked or semi-blocked cold feed.

Title: Re: Who's right ?
Post by thescruff on Jan 13^th, 2018, 11:09pm

[gallery fullsize]thescruff/1515884762.jpg[/gallery]

Assuming the flow/pump is the direction marked, then the problem as said is likely in the circle, eg blocked/restricted cold feed.

Title: Re: Who's right ?
Post by londonman on Jan 14^th, 2018, 2:32am

Does it make any difference if the cold feed is 15mm and the vent 22mm ?

Title: Re: Who's right ?
Post by thescruff on Jan 14^th, 2018, 8:57am

No that is the normal combination, bigger systems could be 22/28.

Title: Re: Who's right ?
Post by londonman on Jan 21^st, 2018, 12:04pm

Just thought I'd give an update and a drawing of the system.

House ch layout

Where it says 'other rads' these are upstairs ones. Flow from heatstore is hidden by the heatstore return pipe.

The radiator marked 'bleed' rad is approximately in the right position and is the one (a) highest in the circuit and (b) furthest rad upstairs from the pump. My plumber called me up yesterday to see how things were going as he has a free day tomorrow. We agreed that I'd try switching off the pump to see what happened. Answer....an airlock when I started it up again.

I have a permanently connected drain cock in place of the bleed valve on the 'bleed' rad. So connect up a hose and open the lockshield. Some air bubbles and then nothing. No more air. No water. Remember the system has been working fine and that radiator was hot and so has water in it. So there's not enough head to push out any water/air in the pipes.. Unless ....unless :idea: you switch the pump off when you bleed, yes ? In that case doesn't the pump act as a block to the flow...which explains why nothing comes out ? But then surely the head in the tank is still acting to push the water out as it's acting back through the flow from the heat store, through the heat store and back up the return to that bleed rad...so it should vent. You have to stick suction on the end of the hose to get it flowing again,whereupon air and water comes out of the hose/lockshield route.

This time was also different in that when I closed the lockshield and opened the flow on the rad, I got a lot of air out. Very confusing

Title: Re: Who's right ?
Post by thescruff on Jan 21^st, 2018, 6:31pm

I'll try and get my head around the drawing.

A, it all looks wrong, so I'll do some imaginary connecting.

What is the blue pipe from the store running alongside the flow, and where how does it connect.

Where does the HW return connect, I assume by HW you mean the cylinder.

The pump should always be off when bleeding.

If there's an airlock say in the flow or return pipe leading to the bleed rad then water will still come out the other pipe so you need to shut one valve to draw the air out. so, if the return is airlocked you need to turn the flow valve off, or vice versa.

Your plumber needs to sort why it's pulling air into the system.

Guessing at this time but it looks like you may have cross connections that are causing a backflow or short circuit

Title: Re: Who's right ?
Post by londonman on Jan 21^st, 2018, 9:59pm

The blue pipe is marked Heat store return. It is the combined return of the CH and cylinder. The HW cylinder return is shown in blue ..it's the pipe that ends below the D in Downstairs.

When I bleed the 'bleed rad' I will turn off the TRV and the lockshield.

1) Then just open the lockshield. See if I can get that flowing again (usually needs suction).

2) Then turn of the lockshield. Turn the TRV on and try to get some flow out of that.

3) Repeat 1 until it all starts working again.

The backflow is VERY interesting. The modification to the CH flow now introduces it halfway down ...see this and earlier diagrams. Now what if he cocked up and that it isn't the original flow that he has tapped into but, in fact, the return ?

Title: Re: Who's right ?
Post by thescruff on Jan 22^nd, 2018, 12:39am

Unfortunately, I don't have enough information to answer your question, you could feel the radiator tails to see which end gets hot first.

What size pipe is the flow from the store and can I assume there are no valves of any description on it.

The first problem is to get the plumber to sort the sucking air in. For what it's worth it should be labour free.

Title: Re: Who's right ?
Post by londonman on Jan 22^nd, 2018, 9:22am

thescruff wrote on Jan 22^nd, 2018, 12:39am:

Unfortunately, I don't have enough information to answer your question, you could feel the radiator tails to see which end gets hot first. Tricky as I did not take any notes as to flow/return on the rads when the original system was running and no guarantee that the original installer got it right in the first place. So I ensured that all the new rads and TRVs were bi-directional. It was your comment re backflow or short circuit that made me wonder about possible mixing up between Flow and Return

What size pipe is the flow from the store and can I assume there are no valves of any description on it. 22mm . Yes, on the Magnaclean

The first problem is to get the plumber to sort the sucking air in. For what it's worth it should be labour free. I agree. For what it is worth, I temporarily capped the end of the vent pipe (to eliminate air being sucked in from there at pump switch on but no cigar

Title: Re: Who's right ?
Post by thescruff on Jan 22^nd, 2018, 10:55am

Valves on the magnaclean are fine.

To be perfectly honest I don't know why you have a vent or neutral zone. The store heating circuit doesn't need it, and I can't see any connection for the boiler to store, which must have a vent or other safety devices elsewhere.

Can you tell me is it a two or one pipe system. You would need to check one of the old rads to tell whether it has 2 pipes serving it or if both tails are connected to the same pipe.

Title: Re: Who's right ?
Post by thescruff on Jan 22^nd, 2018, 11:01am

The longer this goes on the more I think he messed up somewhere.

What is the make, model and size of the store please.

Title: Re: Who's right ?
Post by londonman on Jan 22^nd, 2018, 1:53pm

It's an Akvaterm...2500 litre. But all that is connected is a coil inside it that comes out to the flow and return pipes for the HW cylinder an CH. You are correct in that the Akvaterm and boiler are on a separate circuit with their own open vented system.

We tried feeding the flow into the other CH pipe - just in case the original pipe tapped into wasn't the flow. If anything it made things worse.

He's talking about a more powerful pump to push out any airlock as we are in Catch-22. If we beed the radiator to remove the airlock then that introduces fresh water into the system which will release any dissolved air which will form another airlock which will .....

Title: Re: Who's right ?
Post by londonman on Jan 28^th, 2018, 11:54am

Sorted !

Plumber fitted a more powerful pump yesterday albeit much more expensive. Reasoning went that (a) we knew there was air in the system - from dissolved air in the fresh water being introduced when we hose-pipe bled but (b) the initial pump wasn't man enough to push the air out from wherever it had accumulated.

So...more powerful Grundfos pump and bingo...ran the pump at its highest speed and system worked just fine. Let it get back up to temperature and then switched it off for 30 minutes. Turned it back on and rads warmed up everywhere...upstairs and downstairs. This morning I turned it down to its lower speed and repeated the test. Again..all rads returned to normal working. No hose-pipe bleeding required !

There are also several unexpected bonuses.

1) Any air in the system either goes up the vent pipe or accumulates in the top of that radiator I was using to hose-bleed. A doddle to bleed the air out now.

2) Because it's running the water through faster, means that there is less time for that central main pipe to lose heat in an unwanted area. The rads seem hotter as a result.

3) Saturday morning woke to find the trip in the garage gone. Further inspection revealed that the old circulating pump we'd pressed into service for the boiler primary circuit had burnt out. The pump we took out from indoors has now replaced it ! Just got to take the new one I'd bought Saturday morning back to see if I can get a refund. Plumber called me while I was on my way back...too late to avoid the purchase.

Title: Re: Who's right ?
Post by thescruff on Jan 28^th, 2018, 10:01pm

Did he check the vent wasn't drinking the water from the jar.

Title: Re: Who's right ?
Post by londonman on Jan 29^th, 2018, 2:51pm

It isn't...well, tiniest amount when the pump starts up but what one would expect and certainly only a few cc and not enough to clear the pipe out of water.

Title: Re: Who's right ?
Post by londonman on Feb 2^nd, 2018, 1:31pm

The saga continues.

The good news is that the airlock problem has been sorted. The pump starts up and gets heat to all the rads every time. The only 'funny' as it were is that it makes a lot of noise when it first starts pumping. Almost as if its cavitation or air but then once the water is moving it goes quiet. Annoying, all the same.

The bad news is that we're getting pumping over. I came across this by chance as I popped outside and saw that the header tank was overflowing. When I went into the loft, the water was high and warm. The pump at this time was off.

So I fired up the pump to see what would happen. Lot of suction up the vent pipe, the water level in the header tank then dropped a lot ...enough for the ballcock to drop and let in fresh water.

I let the pump run for a while to ensure everything was at operating temperature and then switched the pump off. Hot water then rushed out of the vent pipe and filled up the header tank...enough for the overflow to kick in. The cycle then repeats. Obviously we are constantly introducing new water into the system which is not good, as you know.

My laymans' guess is that when the pump stops it acts as a brake to the circulating water flow and momentum in that circulating water then rushes up the vent pipe. This happens because the pump is upstairs and so there is not that much head in the vent pipe to stop this happening. If the pump was downstairs then the weight of water in the vent pipe would act as a damper and pumping over wouldn't happen. Is this a reasonable deduction?

So...solution ? I can't see how a bigger tank will help as we'll constantly be introducing air into the circuit by virtue of the water cascading out of the vent pipe in the open air. Sealed system?

Title: Re: Who's right ?
Post by thescruff on Feb 8^th, 2018, 12:10pm

Nothing is sorted and won't be until the plumber goes back to base and gets the pipework re-installed as was.

Water cannot compress so if it's sucking water from the tank, where is it going? the same question in reverse, the tank is overflowing where is the water coming from.

The bigger pump was a bad idea, the pump is designed to the system duty, if it doesn't work then there's a problem with the system.

The plumber needs to look at the fault and work out what he's messed up.

Title: Re: Who's right ?
Post by londonman on Feb 9^th, 2018, 9:58pm

It's quite straightforward if you remember that this is NOT a new system. So there are stubs where some rads used to be and these will contain small amounts of air.

So you fill the system up from the header tank. Filling up ceases. Start the pump. It will compress that air. The header tank will deliver a bit more water into the system. Now the system has more water in it than it should have in a 'steady state'.

Now the pump is turned off by the thermostat. Nothing to compress that air anymore and so that extra volume pushes the water back up the vent pipe and into the header tank. Trouble is that the header tank is very small and not a lot of headroom between normal level and overflow. So it overflows. Then the cycle repeats drawing yet more air into the system.

One..to get rid of wherever the air is stuck will require taking off plasterboard. All over. I ain't going to do that. Simples. We're going to convert the system to a sealed system.